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Fat rewards A new study helps explain why some people find it hard to stick to a low-calorie diet and why eating lots of fat increases your taste for a high-calorie diet.

Neuroscientist Dr Ivan de Araujo, of Yale University, and colleagues, report their findings today in the journal Science.

"People have a hard time sticking to a low-calorie diet," says de Araujo. "We try to identify which signals make low-calorie meals seem so unappealing to the brain."

"We are proposing a mechanism through which the brain becomes faulty in terms of not knowing that a low-calorie meal can also be rewarding."

Previous research has shown that people who eat high-calorie diets or are addicted to drugs are deficient in dopamine, a neurotransmitter in our brain's reward circuit.

"It is believed drug addicts or obese individuals need more drugs or more food respectively to feel satisfied," says de Araujo.

But until now no physiological mechanism linking excessive calorie consumption with a deficient dopamine system has been found.

Lipid messenger

In an experiment using mice, de Arujo and colleagues investigated the role of a lipid in the gut, called oleoylethanolamine (OEA).

This is a naturally occurring substance, formed from fats ingested in the diet, which acts as a messenger between cells.

Previous research has found that animals exposed to a high-calorie diet have low levels of OEA, with the level of suppression matching the suppression of their dopamine system. These animals also show no interest in low-calorie meals.

In this study the researchers confirmed this, and showed that by restoring OEA to the small intestine they could restore the animal's reward circuitry, and this had an impact on their eating preferences.

Their dopamine response increased, as did the animal's interest in low-calorie meals.

In a separate experiment they found that lower fat consumption and higher OEA levels in the small intestine were linked to a decrease in weight.

Gut and brain communication

The findings support the idea that high-fat diets disrupt communication between the gut and the brain via OEA.

"The reward that an animal attributes to a given meal is related to the levels of this lipid in the gastrointestinal tract," says de Araujo.

In their paper, the researchers discuss the pathway that allows the small intestine to communicate with the brain reward circuitry. Apart from OEA binding to receptors in the gastrointestinal tract, the vagus nerve communicates the presence of calories in the gut to the brain.

De Araujo says he can only speculate about implications of the findings for humans at this stage.

"Our paper is not saying that if people take OEA then they will lose weight," he says. "We're talking about a biological system that allows the gut to communicate to dopamine cells."

But he says the results could explain findings from clinical trials that people are more likely to stick with a low-calorie diet supplemented with a chemical that boosts OEA levels in the body.

While to date researchers have thought this is because OEA suppresses appetite, de Araujo says the story is more complex than that.

"OEA increases the sensitivity of the gut to calories," he says. "It's helping the intestine to tell the brain there are calories in their gut, even if it's from low-calorie food. It controls how rewarding a food is to the animal"